This invention relates to a method for casting a metal melt into a continuous metal strip. More particularly, the invention relates to coating the peripheral chill surface of a casting roll with a fluxing agent. The flux coating enhances wetting of the melt to the casting roll and minimizes metal oxide formation on the chill surface during casting of the metal strip.
Direct strip casting involves bringing a melt into contact with a water cooled metal substrate such as a chill surface of a casting roll. This generally is accomplished by casting the melt onto a single casting roll rotating past a refractory pouring nozzle or by pouring the melt into the meniscus formed between a pair of opposing rotating casting rolls. Intimate contact of solidifying metal to a bare metal substrate is required to achieve a high cooling rate. As the solidified metal strip cools while still in contact with the chill surface of the casting roll, the strip contracts. This contraction results in very high tensile stresses due to constraint from the substrate. If adhesion of the strip to the chill surface is too high, the strip may crack. If the adhesion is too low, the strip can lift-off from the chill surface causing a dramatic decrease in the heat transfer rate. This lifting-off can lower the solidification rate and decrease the thickness of the strip. If the lifting-off occurs only in certain portions of the chill surface with intervening areas having good contact, the lifted-off portions experience high tensile stress while those portions of the strip having good adhesion cool more quickly. This differential cooling may result in non-uniform thickness and hot tearing of the strip.
During casting, elements and oxides in the molten metal deposit onto the chill surface as an oxide film. As the thickness of this oxide film increases, the heat transfer rate decreases thereby lowering the solidification rate. This oxide film may also cause gas evolution resulting in porosity in the cast metal strip. This oxide film is tenaciously bonded to the chill surface and difficult to remove by mechanical means such as brushes, buffer wheels, grinding wheels and flapper type disks. These mechanical devices also degrade the chill surface because of uneven wear and tend to vibrate the casting roll thereby disrupting the meniscus.
It is known to condition the peripheral chill surface of a casting roll to prevent strip surface cracks and strip surface quality deterioration due to surface irregularities caused by metal oxide formation. U.S. Pat. No. 5,103,895 relates to twin roll casting of metal strip and discloses copper casting rolls having their chill surfaces electroplated with nickel. Dimples are formed on the chill surfaces. A soluble gas is supplied to the meniscus area between the rolls where the melt comes into contact with the opposing chill surfaces. Gas becomes trapped within the dimples and prevents metal oxide formation on the chill surface. After strip is withdrawn from between the rolls, the chill surfaces of the rolls are cleaned by brushing. Thereafter, the cleaned chill surfaces are roll coated with zircon or alumina to improve the quality of the cast metal strip and to prolong the service life of the casting rolls.
Nevertheless, there remains a need for enhancing wetting of a metal melt to a casting roll and to minimize metal oxide formation on the casting roll during casting of a metal strip. There remains a further need for enhancing wetting of the melt and minimizing oxide formation without leaving grinding lines or gouges on the casting roll and without vibrating the casting roll during casting of the metal strip. Still further, there is a need to provide a layer of relatively low viscosity material which can provide a slip plane between the solidified strip and the casting roll while still maintaining a relatively high heat transfer condition. There is a need to moderate the extremely high rates of heat transfer which can occur at areas of intimate metal-to-metal contact to promote a more uniform overall heat transfer rate. Additional time at elevated temperature also is needed for the solidified strip to mechanically relax in order to prevent the buildup of shrinking stresses.